This invention relates to digital measurements and, more particularly, to a method and apparatus for increasing the dynamic range of digital measurements.
It is typically desirable to measure an analog signal using digital techniques, such as include discrete sampling of the analog signal and analog-to-digital (A/D) conversion, over a wide dynamic range of the analog signal. A limiting factor in applying measurement techniques of conventional systems is uncertainty introduced into the measurement by noise and by A/D conversion, particularly at low analog signal levels, i.e. low signal-to-noise ratio.
Classes of operations in which measurement of an analog signal may be required include: providing an indication of the value or magnitude of the analog signal, such as for a meter or display; and, supplying monitoring and/or protection circuitry, wherein predetermined thresholds are established and conditions above and/or below threshold are used to actuate alarms and/or protective circuitry. Examples of the latter include under/over current or voltage, under/over frequency, under/over temperature, under/over velocity (speed), under/over pressure, over stress, over concentration as in smoke detectors and gas detectors (snifters), or any other variable wherein exceeding and/or operating below a predetermined threshold may be indicated or automatically actuate a response, e.g. switching protective circuitry into operation, flashing lights, signaling with bells, whistles or sirens.
Conventional methods for obtaining digital measurement of an analog signal over a wide dynamic range include: using a linear A/D converter having a large number of bits (e.g. greater than about 10) and designing associated analog circuitry for low noise; compressing the dynamic range of an analog signal using a non-linear amplifier, e.g. logarithmic, supplying the resulting compressed analog signal to an A/D converter and decompressing or expanding the digitized output of the A/D converter, such as with a look-up table; and using an A/D converter for more than one range with range switching performed as a function of input analog signal amplitude by hardware or under software control. All these methods are ultimately limited by noise in the "front end" (i.e. before the A/D conversion circuitry) of the measuring device.
In his experience with monitoring and protection systems, applicant has observed that for many systems, the value of an actual system parameter does not experience large changes during relatively short intervals, especially once the system has reached a steady-state operating condition. It is applicant's intention to use additional time available due to the above-noted observation for increasing the accuracy of system monitoring and protection and also to provide an override scheme for rapidly reacting to any large changes in the actual system parameter which may occur.
Accordingly, it is an object of the present invention to provide a method and apparatus for increasing the dynamic range of digital measurements in order to achieve high accuracy for small analog input signals without sacrificing transient response for large analog input signals.
Another object is to increase the dynamic range of digital measurements by minimizing effects from noise and A/D conversion errors without extensive and/or expensive hardware modification.
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the detailed description taken in connection with the accompanying drawing.